text: Because taking 30 seconds to get up to highway speed would suck ass, and depending on what you are doing with the vehicle (hauling cargo, towing, driving up steep hills etc) you might need the extra power.  Also there are times when it's fine to go as fast as you want l, like if you like taking your car to private race events or tracks.  In fact to keep a small car going highway speed only takes something like 20 horsepower, it's the accelerating that takes the power.  You could theoretically impose laws that require all manufacturers to electronically limit the top speed of any vehicle to 100 or whatever you wanted but people would just remove these anyway.